Heretofore, a fiber reinforced composite material including reinforced fibers such as carbon fibers and glass fibers and a thermoplastic resin such as an epoxy resin and a phenolic resin, has been applied to many fields such as aerospace, automobiles, rail cars, marine vessels, civil engineering and construction and sporting goods since it is lightweight yet is excellent in mechanical characteristics such as strength and rigidity, heat resistance and corrosion resistance. Particularly, in the applications which high performance is required of, a fiber reinforced composite material using continuous reinforced fibers is used. As the reinforced fibers, carbon fibers having excellent specific strength and excellent specific modulus are often used, and as a matrix resin, a thermoplastic resin, particularly, an epoxy resin which has adhesiveness to carbon fibers, heat resistance, elastic modulus and chemical resistance, and is the lowest in cure shrinkage is often used. In recent years, required characteristics of the fiber reinforced composite material become severe as a usage example of the material increases. Particularly, when the fiber reinforced composite material is applied to aerospace applications or structural materials of vehicles or the like, it is required to adequately maintain properties even in a high temperature and/or a high humidity condition. However, although a polymer-based composite material generally has an advantage of light weight, its heat resistance is not high, and particularly heat resistance in a high humidity condition may be insufficient, and therefore applicable use may be limited.
As a curing agent of the epoxy resin, aliphatic amines, aromatic amine compounds, acid anhydrides, and imidazole derivatives are used in combination. Particularly, for aerospace applications which the heat resistance in a high humidity condition is required of, aromatic amine compounds which are excellent in thermal stability of a resin composition and heat resistance of a cured object are often used.
However, since the aromatic amine compound has a lower reactivity with the epoxy resin than other curing agents, prolonged heating at a high curing temperature near 180° C. is required during molding an epoxy resin composition. If the reactivity of the epoxy resin composition is low like this, disadvantages that it takes a long time for molding resulting in high energy cost in molding become obvious. Therefore, a technology which enables the epoxy resin composition to be cured at low temperature in a short time has been desired.
Heretofore, a technique of using a curing accelerator for accelerating curing of the epoxy resin composition is known as one of technologies of curing an epoxy resin in a short time. As such a curing accelerator, phosphorus-based curing accelerators, metal carboxylates, Lewis acids, Brønsted acids and salts of these acids are known, as described in Patent Document 1 or 2.
Further, as other curing accelerators, tertiary amines and salts thereof, imidazoles and salts thereof, and urea compounds are known, as described in Patent Document 3 or 4.